Field of the Invention
The present invention relates to a sheet stacking apparatus configured to stack a plurality of sheets, and a sheet processing apparatus and an image forming system, which include the sheet stacking apparatus.
Description of the Related Art
Hitherto, in a case where a plurality of sheets are placed, at least one edge of the sheets to be placed comes into contact with a position regulating surface formed in a placement portion. At this state, in the vicinity of the position regulating surface with which the sheets come into contact, a curl (warpage) is formed due to an impact of collision between the sheets to be placed and the position regulating surface. Then, a conveyance distance is deviated among the plurality of sheets, with the result that an alignment property is degraded. In view of this, an elastic film, which is arranged in the vicinity of the position regulating surface and inclined with respect to the position regulating surface, is provided to press and eliminate the curl formed in the sheets to be placed (see, for example, Japanese Patent Application Laid-Open No. 2007-76920).
In the above-mentioned pressing method which has hitherto been known, the elastic film is arranged with an inclination, and deformed within a range in which the elastic film interferes with the sheets to be placed, thereby forming a pressing surface configured to press the sheets. Thus, in a case where a small number of sheets which may more frequently cause formation of the curl are placed, the pressing surface of the elastic film is small because a small part of the elastic film interferes with the sheets. On the contrary, in a case where a large number of sheets which may less frequently cause formation of the curl are placed, the pressing surface of the elastic film is large because a large part of the elastic film interferes with the sheets. Thus, there leads to an imbalance state. Therefore, the curl formed in the case of placing a small number of sheets is not sufficiently pressed, with the result that an alignment defect occurs in the sheets to be placed.